The invention relates to an apparatus for improving the fly ash separation in a combustion furnace, particularly in an incinerator with a multiple-flue boiler in which two vertical flues are interconnected by a lower flow-reversing deflector section.
In the case of combustion furnaces with an incorporated steam generator it is not generally possible to position the boiler as a linear vertical unit, i.e., a so-called "single-flue boiler," above the furnace combustion chamber. Therefore, the flue gas path in the furnace is divided up into several vertical flue sections which are interconnected at the ends by a flow-reversing deflector section of, for example, two 90.degree. elbow deflectors or one 180.degree. "U" deflector. At the lower ends of the vertical flues, there are provided 180.degree. deflectors which are at the same time constructed as ash removal hoppers. Due to centrifugal forces, the flue gas flow is separated on flowing through these lower deflections, so that the flue gases flow with locally very high speeds against only one side of the upwardly directed vertical flue. In addition, the centrifugal acceleration of the flue gases leads to the fly ash being carried outwards in the flue gas flow. Relatively large particles of ash, whose size exceeds approximately 200 .mu.m (micro meters) are discharged by centrifugal force into the ash removal hopper by the flue gas flow, which reverses over an approximately semicircular path, whereas the finer ash particles collect in the outer peripheral portion of the reversing flue gas flow. As a result, high fly ash concentrations are formed in the flue gas, so that in the deflection, the zone of high flue gas speed substantially coincides with the zone of high ash concentration. Thus, if in the following upwardly directed flue there are incorporated convective heat exchangers such as boiler superheaters or evaporators as ancillary heating surfaces, the flue gases flow against them in a non-uniform manner, leading to high sooting rates in the area of maximum flue gas speed or fly ash concentration if the fly ash particles have softened by reaching the ash melting point. Admittedly, the sooting of the heating surfaces is smaller in the case of ash with a high melting point, i.e., not-softened fly ash particles, but such ash inturn often causes serious erosion damage to the superheater or evaporator.
Thus, in the case of conventional flue gas deflectors the centrifugal accelerations or forces do not suffice to separate small fly ash particles with a size below 200 .mu.m from the flue gas flow or prevent ash particles with a diameter larger than about 100 .mu.m from reaching the convective heat exchange surfaces arranged in the second flue, as would be desirable. The core of such large flue dust particles is often still soft or plastic, and they violently disintegrate on striking the heating surfaces, leading to the known sooting of the latter. If, however, these ash particles have completely solidified, or have not softened on striking the heat exchange surfaces, then due to their high kinetic energy they cause pronounced erosion which relatively rapidly destroys these heating surfaces in conjunction with corrosion. Furthermore, the one sided flow against the following vertical flue, i.e., the non-uniform action on the heat exchange surfaces incorporated therein, also has a disadvantageous action on the thermal loading of the heat exchanger tubes and the thermal efficiency of the boiler.